Blowdown water is at the same pressure and temperature as water in the boiler drum. Thus by recovering this heat we can save on the blowdown losses.
The boiler feedwater contains suspended particles in the form of impurities and chemicals used for water treatment. These will inevitably collect at the bottom of the boiler in the form of sludge, and are removed by a process known as bottom blowdown.
Additionally the boiler drum water contains dissolved solids. Their concentration increases as the boiler produces steam and consequently the boiler needs to be purged regularly. This purging is achieved during blowdown during which some water is drained and fresh water gets added.
This blowdown water is at the same pressure and temperature as water in the boiler drum. Thus substantial potential for recovering heat exists.
For small process boilers (up to 20TPH), it is possible to recover the heat in the blowdown water by separating the flash steam from the blowdown before draining it. For large boilers (20TPH and above) we can additionally look to recovering the blowdown heat via a heat exchanger. These methods minimize the heat loss through blowdown, saving fuel.
Consider the following case:
Thus in a day the boiler has 2.9TPD of blowdown. This blowdown is at 10.5 bar (190 deg C) and if we flash it at 1.5 bar (120 Deg C), we would have about 370 Kg/day of flash steam.
Returning this flash steam will save 9 tons of fuel per year!